Extended rotation curves of spiral galaxies: dark haloes and modified dynamics

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Abstract
Strict criteria are applied to the sample of spiral galaxies with measured rotation curves in order to select those objects for which the observed rotation curve is an accurate tracer of the radial force law. The resulting sub-sample of 10 galaxies is then considered in view of two suggested explanations for the discrepancy between the luminous mass and the conventional dynamical mass of galaxies: dark haloes and the modified Newtonian dynamics (MOND). This is done by means of least-squares fits to the rotation curves. Three-parameter dark-halo models (M/L for the visible disc, the core radius and the asymptotic circular velocity of the halo) work well in reproducing the observed rotation curves, and it is found that, for the higher luminosity galaxies, the visible matter dominates the mass distribution within the optically bright disc. However, in the low-luminosity gas-rich dwarfs the dark component is everywhere dominant. MOND, with one free parameter, (M/L for the visible disc) generally works well in predicting the form of the rotation curves, in some cases better than multi-parameter dark-halo fits. If the distance to the galaxy is also taken as a free parameter, then the MOND fits are as good as three parameter dark-halo models and, with one exception, the implied distances are consistent with the adopted distances within the probable uncertainty in the distance estimates. Restricting the number of parameter in dark-halo models by making use of the disc-halo coupling does not produce satisfactory fits to the rotation curves. The overall conclusion is that MOND is currently the best phenomenological description of the systematics of the discrepancy in galaxies.

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